Well, technically, they don't so much brainwash as they completely overwhelm with emotion. Need to remember that when... Well, you're L38 I won't spoil who tells you that, but when you talk to someone else they mention how he ALMOST was taken over by Niddhogg, in his moment of weakness he could feel all the rage and hathred of the dragon, emotions which far overwhelmed his mind and he risked forgetting who himself was.
Same thing that happens to a second person, who essentially begs for power in name of revenge in exchange for servitude.

Also need to remember one particular side-quest wherein you need to deal with a heretic that's imprisioned, who's gone downright insane saying his masters demand some sort of sacrifice and he wants you to bring him some... I forget what sort of animal so he can burn it in offering (guard tells you to bring him a rat, though, to shut him up).

Yet, the other heretics you deal with don't behave quite this way, though, they're pretty sane if not... Exactly as much zealots as every Ishgardian is.